The Minishark is a machine gun purchased from the Arms Dealer for 35 gold. If the player has a lot of ammo for it, it can be very useful for taking out pre-hardmode bosses such as the Eye of Cthulhu, the Skeletron and the Eater of Worlds. While being effective in pre-hardmode, this weapon is almost obsolete in hardmode, as all enemies have a lot of defense and it is such a waste to use all the bullets against only a single enemy.
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Terraria - Minishark

Notes

When speaking with the Arms Dealer, he occasionally says "I see you're eyeballin' the Minishark... You really don't want to know how it was made."

The Necro Armor's 20% chance to not consume ammo will stack with the Minishark's 33% chance, yielding a 47% chance that the player will not consume ammo. This means that roughly every second shot will not consume any ammo.

Since the Minishark has 0 knockback, it is not possible to get Unreal prefix with Reforging; the best prefix for the Minishark is Demonic for damage and Murderous for overall stat increase.

Like all machine guns, bullets fired from this weapon have a spread. This can be seen with any type of bullet.

Trivia

The name is likely a reference to the real-life minigun, which is known for having multiple barrels and very fast firerate.